<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title> 盒子模型 </title>
    <style>
        .box{
            width: 300px;
            height: 300px;
            background-color: #ddd;
            border: 10px solid red;
            text-align: center;
            margin: 20px auto;
        }

        .child{
            width: 100px;
            height: 100px;
            color: #ffffff;
            margin: 20px;
            padding: 20px;
            border: 5px solid yellow;
            background-color: blue;

        }
    </style>
</head>
<body>
   <div class="box"> 测试
       <!-- <div class="child">你好</div>  -->
    </div> 
</body>
</html>